In recent years, quad flat packages (hereinafter, will be referred to as QFPs) have been widely used as one form of semiconductor devices using lead frames. FIG. 13 is a sectional view of a typical QFP semiconductor device of the prior art.
As shown in FIG. 13, in the QFP semiconductor device, a semiconductor chip 6 having an IC formed thereon is fixed on the top surface of a die pad 14 with an adhesive 7. Further, a plurality of electrodes (not shown) formed on a surface of the semiconductor chip 6 are connected, via a plurality of wires 8, to a plurality of leads 2 radially disposed around the die pad 14. The semiconductor chip 6, the die pad 14, the wires 8, and the joints of the leads 2 and the wires 8 are integrally molded with a molding resin member 18. The leads 2 are bent into gull-wing shapes outside the molding resin member 18.
In response to multifunctional electronic equipment in smaller sizes and with higher densities, systematic semiconductor components such as a semiconductor device with higher densities and higher functionality have been demanded. In QFP semiconductor devices, the number of pins has been increased, the pitch of leads has been reduced, and heat dissipation has been improved (for example, see “Practical Lesson, VLSI Packaging Technology (II)”, supervised by Susumu Kouyama and Kunihiko Naruse, Nikkei Business Publications, Inc., published on May 31, 1993, pp. 165-170).
Generally, a semiconductor device of high power consumption is configured such that a die pad acting as a radiator plate is exposed from a molding resin member or a radiator plate is separately provided or exposed from the molding resin member, in order to efficiently dissipate heat generated during operations (for example, see Japanese Patent Laid-Open No. 6-291236). FIG. 14 is a sectional view of a QFP semiconductor device including a radiator plate of the prior art.
As shown in FIG. 14, in the QFP semiconductor device, a semiconductor chip 6 having an IC formed thereon is fixed on the top surface of a radiator plate 5 with an adhesive 7. Further, a plurality of electrodes (not shown) formed on a surface of the semiconductor chip 6 are connected, via a plurality of wires 8, to a plurality of leads 2 radially disposed around the semiconductor chip 6 mounted on the radiator plate 5. The radiator plate 5 is bonded to the undersides of the ends of the leads 2 with insulating tape 3 having been die-cut into a predetermined frame shape. The semiconductor chip 6, the radiator plate 5, the wires 8, and the joints of the leads 2 and the wires 8 are integrally molded with a molding resin member 18. The leads 2 are bent into gull-wing shapes outside the molding resin member 18.
In this way, the number of pins has been increased and the pitch of leads has been reduced in QFP semiconductor devices. Thus also in lead frames, leads have been reduced in width and pitch, so that the leads are disadvantageously deformed.
In order to reduce deformations on leads, lead frames have been generally manufactured as follows: first, a metal plate is etched or stamped to form a pattern in which the ends of adjacent leads are connected to each other. Next, after a plating step and a taping step (a step of bonding insulating tape for fixing the leads), the ends of the leads are cut by stamping (for example, see Japanese Patent Laid-Open No. 1-133340).
FIGS. 15 to 18 are main part plan views and process sectional views for explaining the steps of manufacturing the QFP semiconductor device including the radiator plate of the prior art. FIGS. 19 to 21 are process sectional views for explaining the steps of manufacturing the QFP semiconductor device including the radiator plate of the prior art.
First, as shown in FIG. 15, a metal plate is worked by etching or stamping to integrally form a frame (not shown) and the plurality of leads 2 which are connected to the frame and protrude to the center of the frame. At this point, the ends of the adjacent leads 2 are worked to be connected to each other. Before or after this working step, plating is performed on the ends of the leads 2 (portions to be the ends after cutting), the overall metal plate, or a part to be formed into the lead frame on the metal plate.
Next, as shown in FIG. 16, the insulating tape 3 having been die-cut into a predetermined frame shape is bonded to the undersides of the ends of the leads 2 (portions to be the ends after cutting).
After that, as shown in FIG. 17, the connected ends of the leads 2 protruding inward out of the inner frame of the insulating tape 3 are cut by stamping using a cutting die 4, so that the leads 2 are separated from one another. At this point, the leads 2 are fixed with the insulating tape 3 and thus the leads 2 are not separately deformed.
Next, as shown in FIG. 18, the radiator plate 5 having been die-cut into a predetermined shape is bonded to the underside of the insulating tape 3 and is disposed in the frame.
After the steps of FIGS. 15 to 18, a lead frame 1 is completed.
After that, as shown in FIG. 19, the semiconductor chip 6 having a plurality of electrodes (not shown) formed thereon is fixed on the top surface of the radiator plate 5 with the adhesive 7.
Next, as shown in FIG. 20, the plurality of electrodes (not shown) formed on a surface of the semiconductor chip 6 are connected to the plurality of leads 2 via the wires 8.
After that, as shown in FIG. 21, the lead frame 1 is sandwiched by molding dies 9a and 9b, molding resin in a pot 10 is melted, the molten molding resin is injected into the dies with a plunger 11 through a runner 12 and a gate 13, and then the molding resin having been injected into the dies is cured to form the molding resin member.
The leads protruding from the molding resin member are partially cut and bent (not shown) thereafter to complete the QFP semiconductor device.
As has been discussed, in the prior art, a pattern is formed in which the ends of adjacent leads are connected to each other, the insulating tape having been die-cut into a predetermined frame shape is bonded to the undersides of the ends of the leads (portions to be the ends after cutting), and then the connected ends of the leads protruding inward out of the inner frame of the insulating tape are cut.
However, in a QFP semiconductor device in which the number of pins has been increased and the pitch of leads has been reduced, a short circuit caused by wire sweep has become apparent as will be described below:
In the manufacturing process of the lead frame used for the QFP semiconductor device including the radiator plate, insulating tape having been die-cut into a predetermined frame shape is bonded to the undersides of the ends of the leads, and then the ends of the leads are cut by stamping. In the cutting step, when the insulating tape and the leads having a different degree of hardness than that of the insulating tape are cut together, the leads cannot be sharply cut. Thus it is necessary to cut the ends of the leads inside the inner frame of the insulating tape. For this reason, considering the installation accuracy of the lead frame to the cutting die and the die-cutting accuracy and bonding accuracy of the insulating tape, the inner frame of the insulating tape and cut surfaces on the ends of the leads have to be separated from each other at least by about 0.1 mm to 0.4 mm.
Further, in the wire bonding step, the electrodes of the semiconductor chip and the ends of the leads are connected to each other via the wires. In order to securely connect the electrodes and the ends of the leads, it is necessary to securely bond the insulating tape immediately under bonding points. Thus the bonding points of the lead frame including the radiator plate have to be set farther as compared with the bonding points (generally separated from the end sides of leads by about 0.1 mm to 0.5 mm) of a lead frame including a die pad by about 0.1 mm to 0.4 mm.
As has been discussed, in the QFP semiconductor device including the radiator plate, the bonding points on the leads are disposed away from the end sides of the leads. In the resin molding step, the wires are deformed from the upstream side to the downstream side of a resin flow by a pressure of molding resin flowing from the resin injection gate 13 (generally from one of four package corners in a QFP), so that the wires are circularly deformed in plan view as shown in FIG. 22. Thus when the bonding points on the leads are disposed away from the end sides of the leads, as shown in FIG. 23, the wires come close to the ends of the leads adjacent to the wires on the downstream side of a resin flow. When the wires are considerably deformed, a short circuit may occur. This problem is evident particularly in a semiconductor device using a lead frame having multiple pins and leads with a small pitch.